1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of Related Art
An image forming apparatus of an electrophotographic system includes a fixing device. The fixing device applies heat and pressure to a pigment (toner) transferred on a sheet, thereby fixing the pigment. The fixing device includes, for example, a fixing belt supported by two rollers, and a roller (fixing roller) which is provided at the position facing, across the fixing belt, one of the rollers supporting the fixing belt. The fixing device is configured such that a sheet on which toner is transferred passes through a contact portion (nip) between the fixing belt and the fixing roller. The fixing belt or the fixing roller is heated. When the sheet passes through the contact portion between the fixing belt and the fixing roller, the toner on the sheet is heated and pressed, whereby a fixing process is performed.
Conventionally, a layer (elastic layer) made of a material (e.g., rubber) having elasticity is formed on an outer peripheral surface of each roller used in a fixing device, as described in Japanese Patent No. 3654953, and Japanese Patent Application Laid-Open Publication No. 07-139541.
It is desired that a fixing device suitably allows a sheet, to which a fixing process is performed, to be released from a fixing roller. In view of this, there has been a fixing device in which a layer (release layer) which is made of a material (e.g., resin such as tetrafluoroethylene-perfluorovinylether copolymer (PFA)) having a hardness higher than a hardness of an elastic layer is formed to cover an outer peripheral surface of the elastic layer of each roller, in order to facilitate the release of the sheet from the fixing roller.
There has also been a fixing device in which a hardness of an outer peripheral surface of a roller facing a fixing roller is set lower than a hardness of an outer peripheral surface of the fixing roller, in order to increase a nip width.
However, in the conventional fixing device, the roller (fixing roller) having a greater hardness of its outer peripheral surface might damage the roller (roller facing the fixing roller) having relatively a lower hardness of its outer peripheral surface. For example, As shown in FIG. 7, at a portion where an end of the fixing roller in a direction along a rotation axis of each roller is brought into contact with the outer peripheral surface of the facing roller, the outer peripheral surface of the facing roller might be scraped off by the end of the fixing roller (for example, a damaged portion H shown in FIG. 7).
If the width of one of the rollers is set to be larger than the width of the elastic layer of the other roller in the direction along the rotation axis of each roller in order to prevent the end of one of the rollers in the direction along the rotation axis from being brought into contact with the outer peripheral surface of the other roller, the damage on the other roller shown in FIG. 7 is not caused. However, in this case, as shown in FIG. 8 for example, at an end of the elastic layer of the other roller, which is sandwiched between a metal core of the other roller and one of the rollers, the elastic layer projects in the direction of the rotation axis, so that a stress concentration is caused at an interface edge between the elastic layer and the metal core. Accordingly, a crack might be caused in the elastic layer might due to a pressing force at the interface edge. This crack at the interface edge sometimes becomes a starter which makes the crack progress up to a vicinity of a center of the elastic layer.